1. Field of the Invention
This invention relates to a new silacyclohexane compound, a method of preparing it, and a liquid crystal composition which contains it, as well as a liquid crystal display element containing said liquid crystal composition.
2. The Prior Art
A liquid crystal display element utilizes the optical anisotropy and dielectric anisotropy of liquid crystal substances. Display methods include the TN mode (twisted nematic mode), the STN mode (super twisted nematic mode), the SBE mode (super birefringence mode), the DS mode (dynamic scattering mode), the guest-host mode, the DAP mode ("deformation of aligned phase" mode) and the OMI mode (optical mode interference mode). The most common display device has a twisted nematic structure based on the Schadt-Helfrich mode.
The properties required of the liquid crystal substance used in these liquid crystal displays are somewhat different depending on the display method. However, a wide liquid crystal temperature range and stability with regard to moisture, air, light, heat, electric fields, etc., are properties commonly required by all display methods. Furthermore, it is desirable for the liquid crystal material to have a low viscosity, and also to have a short address time, low threshold voltage and high contrast in the cell(s).
Currently, there is no single compound which satisfies all these requirements. In practice, liquid crystal mixtures are used which are obtained by mixing several to more than ten liquid crystal compounds and latent liquid crystal compounds. Because of this, it is also important that the components of a liquid crystal composition mix easily.
Of these components, the following compound has been known as a compound which has both a high T.sub.NI (nematic-isotropic transition temperature) and a high .DELTA.n (anisotropy of the refractive index). ##STR3## (See Japanese unexamined patent publication Tokkai Sho 58-203922.) ##STR4## (See Japanese unexamined patent publication Tokuhyo Hei 3-503651.) ##STR5## (Z.sub.1, Z.sub.2 and Z.sub.3 each independently denote H or F, and X denotes F, CN or Cl.) (See Tokuhyo Hei 3-503771.) ##STR6## (See Japanese examined patent publication Tokko Hei 2-25894.) ##STR7## (See Tokko Hei 2-25894.) ##STR8## (See Tokko Hei 2-25894.) ##STR9## (Z.sub.1, Z.sub.2 and Z.sub.3 each independently denote H or F, and X denotes F, CN or Cl) (See Tokuhyo Hei 3-503771.)
In recent years, along with the expansion of the applications of liquid crystal displays. the characteristics required of liquid crystal materials are becoming more and more advanced. Also, driving methods and operation modes are becoming progressively more diversified.
For automobile onboard use in particular, because of the environment in which they are used, liquid crystal materials with their nematic phase extended to a high temperature range are required. The nematic phase can be extended to a high temperature by adding a liquid crystal compound(s) with a high T.sub.NI (nematic-isotropic transition temperature) as a component.
While the STN mode (super twisted nematic mode) is one of the operational modes which has been known. recent development of the active addressing drive method improved its shortcoming, i.e. slow response time, and this method is being put into practical use. For the liquid crystal material to be used in this method, a liquid crystal material with a high .DELTA.n (anisotropy of the refractive index) is required because the cell gap is made narrower.
On the other hand, the PDLC (polymer dispersion liquid crystal) mode or PNLC (polymer network liquid crystal) mode has been proposed as a new display method and the development is under way toward practical use. Since all of these modes are scattering modes, a liquid crystal material with a large .DELTA.n is required to increase the contrast.